Microelectromechanical beam for allowing a plate to rotate in relation to a frame in a microelectromechanical device

ABSTRACT

An electromechanical device includes a first frame having a first aperture therein, a second frame suspended in the first frame wherein the second frame has a second aperture therein, and a plate suspended in the second aperture. A first pair of beams support the second frame along a first axis relative to the first frame so that the second frame rotates about the first axis. A second pair of beams supports the plate along a second axis relative to the second frame so that the plate rotates about the second axis relative to the frame. The first and second axes preferably intersect at a 90° angle. A first actuator provides mechanical force for rotating the second frame relative to the first frame about the first axis. A second actuator provides mechanical force for rotating the plate relative to the second frame about the second axis. Accordingly, the plate can be independently rotated relative to the first axis and the second axis. Related methods are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 08/719,711, filed on Sep. 27, 1996.

FIELD OF THE INVENTION

The present invention relates to the field of electromechanics and moreparticularly to the field of microelectromechanical devices.

BACKGROUND OF THE INVENTION

Thin film processes developed in the field of microelectronic integratedcircuits have been used to produce precision microelectromechanicaldevices. For example, solid state laser and fiber optic couplings, inkjet nozzles and charge plates, magnetic disk read/write heads, andoptical recording heads have been produced using thin film processesincluding photolithography, sputter deposition, etching, and plasmaprocessing. These thin film processes allow the production ofmicroelectromechanical devices with submicron dimensional control.

One important microelectromechanical device is an electrostaticallydriven rotating mirror which is used in an optical scanner such as a barcode reader. In particular, an electrostatically driven torsionalscanning mirror is discussed in the reference entitled "SiliconTorsional Scanning Mirror" by Kurt E. Petersen, IBM J.Res.Develop., Vol.24, No. 5, September 1980. In this reference, a single-crystal siliconchip contains a mirror element attached to two single-crystal silicontorsion bars. This silicon chip is bonded to another substrate intowhich a shallow rectangular well has been etched. At the bottom of thewell, two electrodes are alternately energized to deflect the mirrorelement in a torsional movement about the silicon torsion bars.

The silicon torsion bars, however, may be unnecessarily stiff thusrequiring excessive torque to rotate the mirror. In addition, thelocation of the electrodes in the path of the rotating mirror mayrestrict the rotation of the mirror. Increasing the distance between theelectrodes and the mirror may reduce the electrostatic force generatedtherebetween. Furthermore, the bonding of the silicon chip to the secondsubstrate may add unnecessary complication to the fabrication of thedevice.

A two-dimensional optical scanner is discussed in the reference entitled"2-Dimensional Optical Scanner Applying a Torsional Resonator With 2Degrees of Freedom" by Yoshinori Ohtuka et al., Proceedings, IEEE MicroElectro Mechanical Systems, 1995, pp. 418, 306-309. This referencediscusses a torsional vibration system where two vibration forces areproduced by one driving circuit. In particular, bimorph cells are usedto excite the torsional vibration. One-dimensional scanning is enabledby driving the bimorph cells with the resonance frequency of either ofthe two torsional vibrations. Two-dimensional scanning can be achievedif the bimorph cells are operated by adding the resonance frequencysignals of the two torsional vibrations. The scanner of this reference,however, may only be able to independently scan in any one dimension atpredetermined resonance frequencies. In other words, because a singledriving circuit is used to excite vibration about two axes, vibrationabout either axis may be limited to predetermined resonance frequencies.The scanner of this reference may also require the assembly of discretecomponents.

Notwithstanding the above mentioned references, there continues to exista need in the art for improved microelectromechanical scanners andmethods.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide improvedelectromechanical devices and methods.

It is another object of the present invention to provide anelectromechanical rotating plate including improved actuators.

It is still another object of the present invention to provide anelectromechanical rotating plate which can reduce the torque needed toeffect rotation.

It is still another object of the present invention to provide anelectromechanical rotating plate which can independently rotate aroundtwo different axes.

These and other objects are provided according to the present inventionby electromechanical devices including a frame having an aperturetherein and a plate suspended in the aperture. A pair of beams extendfrom opposite sides of the plate to the frame wherein a first end ofeach of the beams is fixedly connected to one of the plate and the frameand the second end of each of the beams is in rotational contact withthe other of the plate and the frame so that the plate rotates relativeto the second frame about an axis defined by the beams. Accordingly, theplate is free to rotate about the axis thus requiring relatively littletorque to effect rotation.

Furthermore, the electromechanical devices can include an actuatorhaving an electrode spaced apart from the frame and an arm extendingfrom the electrode to a portion of the plate so that a potentialdifference between the electrode and the frame results in anelectrostatic force which is transmitted by the arm to the plate thuseffecting rotation of the plate. Because this actuator generates anelectrostatic force in response to a potential difference between itselfand the frame instead of the plate, the actuator does not inhibit motionof the plate. In addition, this actuator can provide a biasing supportfor the plate.

According to one aspect of the present invention, an electromechanicaldevice includes a first frame having a first aperture therein, a secondframe suspended in the first aperture wherein the second frame has asecond aperture therein, and a plate suspended in the second aperture. Afirst pair of beams support the second frame along a first axis so thatthe second frame rotates about the first axis. A second pair of beamssupport the plate along a second axis so that the plate rotates aboutthe second axis. The first axis and the second axis preferably intersectat a 90° angle providing independent rotation for the plate about bothaxes. A first actuator provides mechanical force for rotating the secondframe relative to the first frame about the first axis. A secondactuator provides mechanical force for rotating the plate relative tothe second frame about the second axis. Accordingly, the plate can beindependently rotated relative to the first and second axes.

The first and second frames can be formed from a microelectronicsubstrate to provide a microelectromechanical actuator. The plate canalso be formed from this microelectronic substrate. Accordingly, the twoaxis actuator can be fabricated on a single substrate without the needfor wafer bonding. More particularly, the first and second frames andthe plate can be formed from a silicon substrate and the beams can beformed from polysilicon. The microelectromechanical actuator can thus befabricated using thin film processing techniques known in the field ofmicromachining.

Each of the beams supporting the plate can extend from an opposite sideof the plate to the second frame, and a first end of each of the beamscan be fixedly connected to one of the plate or the second frame. Thesecond end of each of the beams can be in rotational contact with theother of the plate or the second frame so that the plate rotatesrelative to the second frame about the axis defined by the beams. Moreparticularly, these beams can be fixedly connected to the plate, andeach beam may include an arched contact surface adjacent the secondframe so that each of the beams rolls on the second frame as the platerotates. The arched contact surfaces further reduce the torque requiredto rotate the plate.

A biasing support can support the plate relative to the second frame sothat the plate and the second frame are coplanar when no mechanicalforce is provided by the second actuator and so that the plate rotatesabout the second axis when mechanical force is provided by the secondactuator. This biasing support can be provided by the actuator. Inparticular, the second actuator can include an electrode spaced apartfrom the second frame and an arm extending from the electrode to aportion of the plate wherein a potential difference between theelectrode and the second frame results in electrostatic force which istransmitted via the arm to the plate thus rotating the plate relative tothe second frame. The electrode can be fixedly connected to the secondframe along a portion thereof spaced from the plate, and the arm can befixedly connected to the plate so that the plate and the second frameare maintained in a common plane when there is no potential differencebetween the electrode and the second frame. Alternately, amicromechanical spring can be provided between the plate and the secondframe.

An insulating layer can be provided between the second frame and theelectrode of the second actuator to prevent electrical shortstherebetween. For example, a silicon nitride layer can be provided onthe second frame. In addition, the arm of the actuator may extend to aportion of the plate closely spaced from the second axis. Accordingly, arelatively small movement of the actuator can result in a relativelylarge rotation of the plate about the second axis.

According to another aspect of the present invention, a method forfabricating an electromechanical device on a substrate includes thesteps of defining plate and frame regions on a face of the substratewherein the frame region surrounds the plate region and wherein theplate region and the frame region are separated by a sacrificialsubstrate region. A supporting structure is formed to support the plateregion along an axis relative to the frame region, and an actuator isformed on the face of the substrate which provides mechanical force tothe plate region. The sacrificial substrate region is then removed sothat the plate region rotates about the axis relative to the frameregion in response to mechanical force provided by the actuator. Thismethod allows the fabrication of a microelectromechanical device with arotating plate using a single substrate thus eliminating the need forwafer bonding.

More particularly, the steps of defining the plate and frame regions mayinclude doping the respective regions, and the step of removing thesacrificial substrate region may include etching undoped portions of thesubstrate. Accordingly, the plate and frame regions can be defined earlyin the fabrication process and then separated later in the fabricationprocess after forming the beams and the actuators. Accordingly, theplate and frame regions can be defined without creating significanttopography allowing the beams and actuators to be formed on a relativelyflat substrate.

The step of forming the supporting structure can include the steps offorming a pair of beams on opposite sides of the plate region whichdefine an axis of rotation through the plate region. Each of the beamsextends from the plate region to the frame region, and each of the beamsis fixedly connected to one of the plate region and the frame region. Asecond end of each of the beams is in rotational contact with the otherof the plate region and the frame region so that the plate rotatesrelative to the frame. As discussed above, the rotational contactreduces the torque required to rotate the plate.

The step of forming the beams can include the steps of forming asacrificial layer on the substrate, forming first and second holes inthe sacrificial layer exposing portions of the plate region along theaxis, and forming first and second partial holes in the sacrificiallayer opposite the frame region without exposing the frame region. Thepartial holes are formed along the axis, and the partial holes can beformed by isotropically etching the sacrificial layer. First and secondbeams are formed on the sacrificial layer wherein each of the beams isfixedly connected to the plate region through a respective one of theholes in the sacrificial layer. Each beam extends from a respectiveexposed portion of the plate region to a respective partial holeopposite the frame region. The sacrificial layer is then removed so thatthe first and second beams extend from the plate to the frame in acantilevered fashion. Accordingly, each of the beams includes an archedcontact surface in rotational contact with the frame.

The step of forming the sacrificial layer may include the steps offorming a first sacrificial sublayer having a first etch rate andforming a second sacrificial sublayer having a second etch rate which ishigh relative to the first etch rate. The step of isotropically etchingthe sacrificial layer thus forms the partial holes primarily in thesecond sacrificial sublayer. The first sacrificial sublayer with therelatively low etching rate thus ensures an adequate spacing between thecontact surface of the beam and the substrate.

The step of forming the actuator can include the steps of forming anelectrode spaced apart from the frame region and an arm extending fromthe electrode to a portion of the plate region. A potential differencebetween the electrode and the frame region results in electrostaticforce which is transmitted by the arm to the plate region. Accordingly,the plate can rotate in response to the electrostatic force generatedbetween the electrode and the frame. Furthermore, by providing a fixedconnection between the arm and the plate, the actuator can provide abiasing support which supports the plate relative to the frame so thatthe plate and the frame are coplanar when no mechanical force isprovided to the plate.

Electromechanical devices of the present invention can thus provideindependent rotation of the plate about two axes of rotation. The beamswhich provide a rotational contact between the plate and the frame canreduce the torque required to rotate the plate. Furthermore, theelectrostatic actuators which generate a mechanical force in response toa potential difference between the electrode and the frame need not liein the path of rotation of the plate. Electromechanical devices of thepresent invention can also be fabricated on a single substrate usingmicromachining techniques.

By providing a reflecting surface on the plate, a rotating mirror for ascanner can be produced. Accordingly, a rotating mirror can be producedefficiently and economically without the need for wafer bonding or theassembly of discrete components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view of a substrate with doped regions defining firstand second frame regions and a plate region according to the presentinvention.

FIG. 1B is a cross sectional view of the substrate of FIG. 1A takenalong the section line labeled FIG. 1B.

FIG. 2A is a plan view of the substrate of FIG. 1A covered with apatterned sacrificial layer.

FIG. 2B is a cross sectional view of the substrate and sacrificial layerof FIG. 2A taken along the section line labeled FIG. 2B.

FIG. 2C is a cross sectional view of the substrate and sacrificial layerof FIG. 2A taken along the section line labeled FIG. 2C.

FIG. 3A is a plan view of the substrate and sacrificial layer of FIG. 2Awith supporting beams formed thereon.

FIG. 3B is a cross sectional view of the substrate of FIG. 3A takenalong the section line labeled FIG. 3B.

FIG. 3C is a cross sectional view of the substrate of FIG. 3A takenalong the section line labeled FIG. 3C.

FIG. 4A is a plan view of the substrate of FIG. 3A with electrostaticactuators formed thereon.

FIG. 4B is a cross sectional view of the substrate of FIG. 4A takenalong the section line labeled FIG. 4B.

FIG. 4C is a cross sectional view of the substrate of FIG. 4A takenalong the section line labeled FIG. 4C.

FIG. 5A is a plan view of the substrate of FIG. 4A after removing thesacrificial layers and sacrificial portions of the substrate.

FIG. 5B is a cross sectional view of the substrate of FIG. 5A takenalong the section line labeled FIG. 5B.

FIG. 5C is a cross sectional view of the substrate of FIG. 5A takenalong the section line labeled FIG. 5C.

FIG. 5D is an alternative cross sectional view of the substrate of FIG.5A taken along section line labeled FIG. 5B according to one embodimentof the present invention.

FIG. 6 is a perspective view of an optical scanner according to thepresent invention.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, the thicknesses of layers and regions are exaggerated forclarity. Like numbers refer to like elements throughout.

A top view of a microelectromechanical rotating mirror according to thepresent invention is illustrated in FIG. 5A, while cross sectional viewsare illustrated in FIGS. 5B and 5C. In particular, a first frame 50 anda second frame 52 respectively surround the rotating plate 54 on which amirror can be formed. A first pair of beams 56 support the second frame52 along a first axis relative to the first frame 50 so that the secondframe 52 rotates about the first axis relative to the first frame 50. Asecond pair of beams 58 supports the plate 54 along a second axisrelative to the second frame 52 so that the plate 54 rotates about thesecond axis relative to the second frame 50. As shown, the first axis ofrotation and the second axis of rotation intersect at a 90° angle.

A first set of four actuators 60 is provided on the first frame 50 withone actuator on each side of each of the beams 56 of the first pair.These actuators 60 provide mechanical force for rotating the secondframe 52 relative to the first frame 50 about the first axis which isdefined by the first pair of beams 56. A second set of four actuators 62is provided on the second frame 52 with one actuator on each side ofeach of the beams 58 of the second pair. These actuators 62 providemechanical force for rotating the plate 54 relative to the second frame52 about the second axis which is defined by the second pair of beams58. In addition, both sets of actuators assist in positioning andsupporting the movable plate and second frame. Accordingly, the platecan rotate independently about both the first axis of rotation and thesecond axis of rotation.

As shown in cross section in FIG. 5B, each of the beams 58 of the secondpair extends from a side of the plate 54 and is fixedly connectedthereto. Each beam 58 extends over the second frame 52 and is positionedadjacent the surface thereof for rotational movement. Accordingly, thesebeams 58 define the second axis of rotation about which the plate 54rotates relative to the second frame 52. Because the beams 58 are notfixedly connected to both the plate 54 and the second frame 52, thetorque required to rotate the plate about the second axis can bereduced. Furthermore, the arched contact surface 64 of each beam 58adjacent the second frame 52 allows the beam to roll on the second frameduring rotation of the plate 54 further reducing the torque required torotate the plate.

As will be understood by one having skill in the art, the arched contactsurface 64 can be rounded, pointed, or otherwise shaped to provide arolling motion for the beam when the plate rotates. For example, withreference to FIG. 5D, in one embodiment, the arched contact surface 64is pointed. In addition, the pair of second beams extending fromopposite sides of the plate is defined to include a structure whereinthe pair of beams are joined on the plate. In other words, the pair ofsecond beams can be provided by a structure on the plate which extendsacross the plate and beyond opposite sides of the plate. The pair offirst beams can be provided by a similar structure on the second frameraised to permit movement of the plate. Alternately, the pair of secondbeams and the plate can be formed from a single layer of a material suchas polysilicon so that the pair of second beams and the plate areconfined within a single plane. Again, the pair of first beams and thesecond frame can similarly be formed from a single layer. The beams 56extending from the second frame 52 operate in the same manner asdiscussed above with regard to the beams 58 extending from the plate 54.

As shown in FIG. 5C, each of the actuators 62 includes at least oneelectrode 66 spaced apart from and electrically insulated from thesecond frame 52, and an arm 68 extending from the electrode 66 andattached to a portion of the plate 54 off the second axis. The electrodeis an electrically conductive plate generally parallel to the secondframe and spaced from it by supports, as illustrated in FIGS. 5B and 5C.The supports are preferably located at the edge of the electrode and arelocated on two sides, but this configuration may change as necessary ordesirable to obtain the optimum combination of structural support andflexibility for movement.

Accordingly, a potential difference between the electrode 66 and thesecond frame 52 will result in an electrostatic force which istransmitted via the arm 68 to the plate 54 thus rotating the plate 54relative to the second frame 52. By attaching the arm 68 to the plate54, the plate 54 and the second frame 52 can be maintained in agenerally common plane when there is no potential difference between theelectrode 66 and the second frame 52. The actuator 62 can thus provide astructure that supports the plate 54 relative to the second frame 52 andselectively biases it to induce desired rotation. Such biasing supportcan alternately be provided by micromachined springs which can be formedfrom the same material used to form the plate and the second frame. Forexample, serpentine springs may be formed between the plate and frame.

The rotation of the second frame 52 is illustrated by arrows 59 in FIGS.5B and 5C. This rotation occurs in the plane of FIGS. 5B and 5C aboutthe axis defined by the first pair of beams 56 (shown in FIG. 5A). Theplate 54 rotates into and out of the plane of FIGS. 5B and 5C about theaxis defined by the second pair of beams 58.

By generating a potential difference between the electrode 66 and thesecond frame 52 at a location remote from the plate or its rotationalarc, the electrode does not interfere with or intrude into the path ofrotation of the plate 54. Accordingly, the electrode 66 can be closelyspaced from the second frame 52 thus increasing the electrostatic forcegenerated without reducing a range of motion for the plate 54.Furthermore, the useful size of the electrode 62 and the electrostaticforces generated thereby are not limited by the size of the plate 54.

The arm 68 preferably extends to a portion of the plate 54 closelyspaced from the second axis. Accordingly, a relatively smalldisplacement of the arm 68 can result in a relatively large rotation ofthe plate 54. As shown, the actuator arms 68 are fixedly connected tothe plate 54 thus providing biasing support for the plate.Alternatively, the arms can extend adjacent to the plate without beingfixedly connected thereto. Accordingly, the torque required to rotatethe plate can be reduced because the arms are positioned adjacent thesurface of the plate but are not attached to it. The insulating layer110 forming the upper surface of the second frame can be used to preventelectrical shorts between the electrode 62 and the conductive portion ofthe second frame 52. The actuators 60 on the first frame includingelectrodes 70 and the arms 72 operate as discussed above with regard tothe actuators 62 on the second frame.

By providing a reflective surface 107 on the plate, a rotating mirror isproduced. This rotating mirror can be used to provide an optical scanner200 such as a bar code reader, as shown in FIG. 6. For example, a laser202 or other energy source can generate a beam of electromagneticradiation 204 such as light and project the beam onto the reflectivesurface 107 of the rotating mirror. By rotating the mirror about thefirst and second axes, the reflected beam 206 can be scanned in apredetermined pattern. This scanned beam can be used to read a patternsuch as a bar code. The control circuit 208 may provide control signalswhich control the operation of the rotating mirror and the operation ofthe laser.

A method for fabricating the microelectromechanical rotating mirror ofFIGS. 5A-C will be discussed as follows with reference to FIGS. 1A-C,2A-C, 3A-C, 4A-C, and 5A-C. As shown in FIGS. 1A-C, predeterminedsurface regions of substrate 100 are doped thus defining the first frameregion 102, the second frame region 104, and the plate region 106. Thesubstrate can be a microelectronic substrate formed from materials suchas silicon, gallium arsenide, or other materials used in the fabricationof microelectronic devices. The predetermined surface regions can bedoped with boron by either an implant or a diffusion step. Each of theseregions is separated by sacrificial substrate regions 108. The dopantcan later serve as an etch stop so that the sacrificial regions of thesubstrate can be selectively etched away leaving only doped portions ofthe substrate. Such an etch can be performed at a later point infabrication to separate the first frame 50, the second frame 52, and theplate 54 as shown in FIGS. 5A-C. Accordingly, the frame and plateregions can be defined without creating significant topology which couldincrease the difficulty of processing.

A protective nitride layer 110 can be formed on the doped regions of thesubstrate, and a sacrificial layer 112 can then be formed on thesubstrate and patterned, as shown in FIGS. 2A-C. The nitride layer 110can provide stress compensation in the frame and plate regions whenseparated, and the nitride layer 110 can also provide an insulatinglayer between the electrodes and respective frames. The nitride layer110 can also provide insulation between conductive lines and the dopedregions of the substrate. Alternately, the nitride layer 110 can coveronly portions of the doped regions as required. For example, portions ofthe plate could be left uncovered using the reflective properties of thesubstrate to provide the mirror. A protective nitride layer 111 can alsobe formed on the back of the substrate 100. The nitride layers 110 and111 can be formed simultaneously.

Partial holes 126 in the sacrificial layer provide a mold for the archedcontact surfaces for each of the first pair of beams, and the partialholes 128 provide a mold for the arched contact surfaces for each of thesecond pair of beams. The partial holes 126 and 128 can be formedsimultaneously by isotropically etching partial holes through thesacrificial layer 112 without exposing the substrate. The isotropic etchprovides the arched surfaces shown in FIG. 2B. In particular, smallportions of the sacrificial layer 112 are exposed photolithographically,and a wet isotropic etch is performed for a predetermined time so thatthe partial hole is formed with the arched surface and without exposingthe substrate.

The sacrificial layer 112 can include a first sacrificial sublayer 112Ahaving a first etch rate and a second sacrificial sublayer 112B having asecond etch rate which is high relative to the first etch rate.Accordingly, the partial hole is formed primarily in the secondsacrificial sublayer 112B with the first sacrificial sublayer 112A beingused to prevent the substrate from being exposed. In particular, thefirst sacrificial sublayer 112A can be formed from a thermal siliconoxide, and the second sacrificial sublayer 112B can be formed fromphosphorus silicate glass (PSG) which has an etch rate that is high whencompared to that of a thermal silicon oxide. Accordingly, a portion ofthe sacrificial layer remains between the arched surface of the partialholes and the substrate. Dashed lines in FIG. 2A indicate the frame andplate regions of the substrate which have been previously defined andcovered with the sacrificial layer 112.

The sacrificial layer 112 can then be patterned to expose portions ofthe substrate to which the actuators and supporting beams will beanchored. The holes 114 expose portions of the substrate to which thefirst pair of beams will be anchored to the second frame. The holes 116expose portions of the substrate to which the second pair of beams willbe anchored to the plate. The holes 118 expose portions of the substrateto which the first set of actuator electrodes will be anchored to thefirst frame, and the holes 120 expose portions of the substrate to whichthe first set of actuator arms will be anchored to the second frame. Theholes 122 expose portions of the substrate to which the second set ofactuator electrodes will be anchored to the second frame, and holes 124expose portions of the substrate to which the second set of actuatorarms will be anchored to the plate. Preferably, the step of forming thepartial holes 126 and 128 precedes the step of forming the holes 114,116, 118, 120, 122, and 124 which expose the substrate because higherresolution patterning may be required to form the partial holes.

As shown, the actuator electrodes can be anchored to the substrate alongL shaped patterns 118 and 122 as shown in FIG. 2A. Alternately, theactuator electrodes can be anchored to smaller portions of the substratesuch as patterns including only the linear portion of the Lperpendicular to the respective axis of rotation. The larger L shapedanchor may provide a stiffer actuator capable of providing lower forceat a higher frequency of operation, while the smaller linear anchor mayprovide a more flexible actuator capable of providing greater force at alower frequency of operation.

A polysilicon layer is formed on the patterned sacrificial layer 112 andpatterned to form the beams which define the axes of rotation and toform the anchoring structures for the actuator electrodes and arms, asshown in FIGS. 3A-C. In particular, each of the beams 56 fills arespective hole 114 fixedly connecting it to the second frame region ofthe substrate and extends to the respective partial hole 126 thusforming the arched contact surface spaced from the first frame region ofthe substrate. Each of the beams 58 fills a respective hole 116 fixedlyconnecting it to the plate region of the substrate and extends to therespective partial hole 128 thus forming the arched contact surfacespaced from the second frame region of the substrate.

The anchoring structures 130, 132, 134, and 136 are also formed fromthis patterned layer of polysilicon. Anchoring structures 130 are usedto anchor the electrodes of the first set of actuators to the firstframe region of the substrate, and anchoring structures 132 are used toanchor the electrodes from the second set of actuators to the secondframe portion of the substrate. Anchoring structures 134 are used toanchor the arms from the first set of actuators to the second frameregion of the substrate, and anchoring structures 136 are used to anchorthe arms from the second set of actuators to the plate region of thesubstrate. As discussed above, the relatively large L shaped anchoringstructures 130 and 132 can be used to provide relatively stiffactuators. Alternately, only the linear portions of the anchoringstructures 130 and 132 perpendicular to the respective axes of rotationcan be used to provide a more flexible actuator. Again, the dashed linesof FIG. 3A indicate the defined portions of the substrate which havebeen covered by the sacrificial layer 112.

A second sacrificial layer 140 is then formed on the structure of FIGS.3A-C and patterned, and a second patterned polysilicon layer is formedthereon as shown in FIGS. 4A-C. The second sacrificial layer 140 ispatterned to expose the anchoring structures 130, 132, 134, and 136shown in FIGS. 3A-3C. Because the polysilicon beams 56 and 58 arecovered by the second polysilicon layer 140, these beams are illustratedwith dotted lines in FIG. 4A. As before, the defined regions of thesubstrate are also illustrated with dotted lines.

The second patterned polysilicon layer forms the actuators 60 includingelectrodes 70 and arms 72, and the actuators 62 including electrodes 66and arms 68. The second patterned polysilicon layer can be heavily dopedso that the electrode portion of the actuator is conductive. As shown inFIGS. 4B and 4C, the actuators 60 and 62 are formed on the respectiveanchoring structures 130, 134, 132, and 136 which are exposed by thesecond sacrificial layer. By forming the beams 56 and 58 from the firstpolysilicon layer, and forming the actuators 60 and 62 from the secondpolysilicon layer, the beams can have a thickness that is different fromthat of the actuators. Preferably, the first polysilicon layer isrelatively thick so that the beams 56 and 58 are stiff, and the secondpolysilicon layer is relatively thin so that the electrodes of theactuators are relatively flexible. For example, the beams can be formedfrom a polysilicon layer on the order of several microns thick, and theelectrodes can be formed from polysilicon on the order of less than onemicron thick. Alternately, the beams and the electrodes can be formedfrom the same polysilicon layer thus eliminating the need to form andpattern the second sacrificial layer and the second polysilicon layer.

The spacing between the actuator electrode and the substrate isdetermined by the combined thicknesses of the sacrificial layers.Accordingly, this spacing can be precisely controlled, and very smallspacings can be provided.

The sacrificial layers and the sacrificial portions of the substrate arethen selectively removed to form the microelectromechanical rotatingmirror shown in FIGS. 5A-C. The backside nitride layer 111 is patternedto provide a mask for etching the substrate 100 using an etchant such asKOH to remove the undoped and unmasked portions of the substrate.Accordingly, the second frame region 104 is separated from the firstframe region 102, and the plate region 106 is separated from the secondframe region 104, thus forming the first frame 50, the second frame 52,and the plate 54. The sacrificial layers 112 and 140 are thenselectively removed using an etchant such as HF to free the cantileveredactuator arms and support beams. As shown, the second frame 52 issuspended relative to the first frame 50 by the actuators 60. The plate54 is suspended relative to the second frame 52 by the actuators 62.

The beams 58 thus include arched contact surfaces 64 adjacent the secondframe 52 so that each beam is in rotational contact with the secondframe. As shown, when none of the actuators is activated, the contactsurfaces of both of the beams can be slightly spaced from the secondframe 52. When force is applied to the plate by one or more of theactuators 62, the contact surface comes into contact with the secondframe forcing the plate to then rotate about the axis defined by thebeams. Accordingly, the beam rolls relative to the frame reducing thetorque required to rotate the plate. The beam is thus defined as beingin rotational contact with the frame even though a narrow space mayexist between the contact surface of the beam and the frame when noforce is applied to the plate.

By anchoring the arms 68 of the actuators 62 relatively close to theaxis defined by the beams 58, a relatively small movement of the armwill result in a relatively large rotation of the plate. Accordingly,the electrodes of the actuators can be closely spaced from the secondframe thus increasing the electrostatic force generated thereby whilestill effecting a significant rotation of the plate.

While the actuators are illustrated with the rotating beams, theactuator can alternately be used with other means for defining the axisof rotation. For example, the actuator can be used with torsion barsand/or a supporting ridge. Conversely, while the rotating beams areillustrated with the electrostatic actuators, the rotating beams can beused with other actuators. For example, the rotating beams can be usedtogether with thermal actuators, magnetic actuators, piezoelectricactuators, and bimetallic actuators.

The plate 54 can serve as the mirror. If the substrate is a polishedsingle crystal semiconductor material, a mirror finish can be providedby removing a portion of the nitride layer therefrom. Alternately, alayer 107 of a reflective material such as a metal can be formed on theplate. According to yet another alternative, the plate can be formedfrom the first polysilicon layer used to form the beams and polished orotherwise provided with a reflective layer thereon. The plate and thebeams can thus be formed as an integrated structure.

Electrical connections on each of the first frame, the second frame, andthe plate can be provided by conductive lines thereon. For example,metal lines or doped polysilicon lines can provide interconnection, andthese lines can be insulated from the doped silicon by the nitride layer110. Electrical connection between the first and second frames can beprovided by wire bonding, through electrical connection across the beams56, through electrical connection across the actuator arms 72, orthrough electrical connection across a long flexible bridge structurebetween the first and second frames.

The microelectromechanical rotating mirror of FIGS. 5A-C can thus beformed on a single substrate without the need to bond wafers or assemblediscrete components. Accordingly, this mirror can be economically andreliably fabricated. Furthermore, the mirror provides independentrotation about two axes, and the electrodes do not lie in the path ofrotations of the mirror.

By using a doping technique to define the frame and plate regions of thesubstrate, subsequent processing can be performed on a smooth substrate.The subsequent processes can be performed more easily due to thereduction in topography. Significant topography is added to thestructure only after the final etches removing the sacrificial layersand substrate regions which occur after most processes which aresensitive to extreme topography have been completed.

In the drawings and specification, there have been disclosed typicalpreferred embodiments of the invention and, although specific terms areemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being set forthin the following claims.

What is claimed is:
 1. A microelectromechanical beam for supporting aplate and allowing the plate to rotate in relation to a frame, whereinsaid beam comprises:a first end having an arched contact surface forcontacting the frame; and an opposed end connected to the plate, suchthat the arched contact surface of said first end allows the plate torotate in relation to the frame with reduced rotational torque.
 2. Amicroelectromechanical beam according to claim 1, wherein said archedcontact surface is spaced apart from the frame such that said archedcontact surface only contacts the frame when a force is exerted on theplate pushing the plate toward the frame.
 3. A microelectromechanicalbeam according to claim 1, wherein said arched contact surface isconnected to said frame such that the plate and frame are connected, andwherein said arched contact surface allows the plate to rotate inrelation to the frame with reduced rotational torque.
 4. Amicroelectromechanical beam according to claim 1, wherein said archedcontact surface of said beam is pointed.